1. Field of the Invention
The invention generally relates to a shock absorbing rear wheel suspension apparatus for a bicycle, and more particularly to a rear wheel suspension having a controlled variable shock rate.
2. Description of the Prior Art
A variety of rear wheel suspension systems have been used on bicycles to improve performance. Many of these suspension systems are complicated structures involving a plurality of frame members, linkages of the swingarm frame members to other bicycle frame members and the installation of a shock absorber within the frame members to control the motion of the swingarm and the rear wheel engaged therewith.
One such prior art suspension system is disclosed in U.S. Pat. No. 6,361,059 which describes a bicycle with a rear wheel suspension having a single pivot axis that is located proximate the bottom bracket of the bicycle frame. A top portion of the swingarm, proximate the seat stays, is engaged to a shock absorber which controls the motion of the rear wheel swingarm. Another prior art suspension system is disclosed in U.S. Pat. No. 6,206,397 in which a bottom portion of the swingarm is engaged by a first linkage proximate the bottom bracket, and an upper portion of the swingarm proximate the seat stay is engaged with a second linkage proximate the top tube of the bicycle frame. A shock absorber is engaged to the swingarm to control the motion of the swingarm and therefore the rear wheel of the bicycle.
In analyzing the performance of bicycle frames including those disclosed in the prior art, a comparison can be made between the vertical wheel travel distance that the rear wheel moves upward (AVWT) and the change in the length of the shock absorber (ASL). The ratio of these two measurements, the change in the shock absorber length divided by the change in vertical rear wheel travel (ASL/AVWT) is called the shock rate (SR). In various bicycle frame designs, the shock rate can be approximately constant, it can increase as the vertical wheel travel increases or it can decrease as the vertical wheel travel increases.
Controlling the shock rate can provide improved bicycle performance and the present invention is directed to a bicycle rear wheel suspension system in which the shock rate is variable and controllable to provide improved bicycle performance characteristics.